1. Technical Field
The present invention relates generally to a visible light communication method and apparatus and, more particularly, to modulation and demodulation technology for the transmission and reception of visible light communication (VLC) that is capable of supporting the dimming function of controlling the brightness of light-emitting diode (LED) lighting in visible light communication.
2. Description of the Related Art
Recently, due to abrupt changes in the environment that require reductions in the consumption of energy and the production of greenhouse gas, lighting equipment and display devices using LEDs have rapidly spread and have a lot of uses in daily life, such as in automobiles, signal lights, signboards, TV, monitors, mobile devices, special lighting, and general lighting. Furthermore, research has been actively carried out into wireless communication technology that attempts to combine both the intrinsic purpose of LED light sources and the purpose of communication means by adding communication functionality to LED lighting equipment or LED display devices.
LED light sources are widely used because they have the advantages of having longer life spans than existing light sources, having superior power efficiency, enabling various colors to be implemented, and allowing digital control. Furthermore, with the enactment of the Kyoto Protocol to the United Nations Framework Convention on Climate Change, it became necessary to reduce the consumption of energy and the production of greenhouse gas, and this is one of the reasons why various countries over the world are rapidly replacing existing light sources with LED light sources.
Visible light communication technology is wireless communication technology that wirelessly transfers information using light in the visible wavelength band, which can be perceived by the eyes of humans. This technology is distinguished from existing wired optical communication technology in that the former technology utilizes light in the visible wavelength band.
Furthermore, unlike Radio Frequency (RF) wireless communication technology currently being widely used, visible light communication technology has the convenience of freely using corresponding frequencies without restriction or permission, provides superior physical security, and has the advantage of enabling a user to perceive a communication link with his or her eyes. In particular, visible light communication technology is convergence technology that is capable of achieving both the intrinsic purpose of light sources and communication functionality.
Since a visible light communication system is a system that transmits and receives information based on a visible light, wireless communication should be performed in the state in which the basic functions of lighting have been fulfilled. One of the basic functions of lighting is the function of controlling the brightness of lighting, that is, a dimming function, and thus a visible light communication system should support both the function of performing wireless communication using visible light and a dimming function. Technologies that have been proposed to control the brightness of lighting in connection with visible light communication include an amplitude dimming method and a variable-pulse position modulation (VPPM) dimming method.
Amplitude dimming is a method of controlling the brightness of a light source by changing the amplitude of a signal in an On Off Keying (OOK) modulation method, and VPPM dimming is a method of providing the function of controlling brightness using a VPPM modulation method.
As described above, amplitude dimming is a method of controlling the brightness of a light source by changing the amplitude of a signal in an OOK modulation method. FIGS. 1 to 3 illustrate an amplitude dimming method when Manchester code and an OOK modulation method have been employed.
FIG. 1 illustrates an optical waveform output from an LED light and a corresponding average output when Manchester-OOK technology has been applied to a transmission unit. This indicates that the average output is the same as an optical output that is generated when a Direct Current (DC) signal having an amplitude equal to ½ of the amplitude PM of a Manchester-OOK signal is applied.
Meanwhile, FIG. 2 shows that the average output of an LED light source can be increased by operating the LED light source so that the amplitude PM1 of a Manchester-OOK signal is wider than the signal amplitude PM of FIG. 1. Similarly, the average output can be decreased by operating the LED light source so that the amplitude of a signal is narrow.
Furthermore, FIG. 3 shows that although the amplitude PM2 of a Manchester-OOK signal is the same as the signal amplitude PM of FIG. 1, an average output can be increased as a result by applying a DC offset. However, in practice, it is difficult for such amplitude dimming technology to fulfill the maximum brightness level that is provided by an LED light having no visible light communication function and having the same specifications. The reason for this is that a signal having an amplitude significantly above the allowable range of an LED light source should be applied, in which case applied instantaneous signal amplitudes may damage an LED light source, which results in a significant reduction in the life span of the LED light source in the long term. Furthermore, controlling brightness by changing the amplitude of a signal means that the intensity of current to be supplied to an LED light source is changed in response to the control of brightness, with the result that amplitude dimming has the possibility of causing color variation in the LED light source.
A VPPM modulation method is a modulation method that was devised from a double pulse position modulation (2-PPM) modulation method and a pulse width modulation (PWM) modulation method in order to prevent flickering from occurring inside a frame and to control the brightness of a light source, and is one of the modulation methods for visible light communication that were adopted in the IEEE 802.15.7 international standard.
A 2-PPM modulation method is a modulation method that represents bit 0 and bit 1 according to the locations of pulses, as illustrated in FIG. 4, and provides the same average brightness for both bit 1 and bit 0, as in the optical output of Manchester code. Accordingly, this technology can prevent flickering from occurring inside a frame.
Meanwhile, a PWM modulation method is a modulation method that controls the brightness of a light source by changing the widths of pulses, as illustrated in FIG. 2, and is currently being widely used in LED lighting.
Meanwhile, a VPPM modulation method is similar to a 2-PPM modulation method in that bit 0 and bit 1 are represented according to the locations of pulses, and is similar to a PWM modulation method in that the widths of pulses can be changed in accordance with brightness desired by a user.
Therefore, an optical waveform having a pulse width of 50%, which is modulated using variable-PPM technology, is the same as a 2-PPM modulation waveform, as illustrated in FIG. 5.
Furthermore, FIG. 5 illustrates a mechanism for controlling brightness using a VPPM modulation method, and illustrates an example in which when a digital signal of 001 is modulated using a variable-PPM technology, even the same data exhibits brightness that varies in response to a change in pulse width.
In addition, VPPM technology subdivides each change in pulse width into smaller increments, and thus can provide brightness similar to the maximum brightness that is provided by an LED light having the same specifications.
Furthermore, brightness is controlled by pulse widths along a time axis, rather than the amplitudes of pulses. Therefore, an LED light source may not be damaged and color variation in the light source may not also occur.
Korean Patent Application Publication No. 2010-0060900 discloses a method for enabling visible light communication without deteriorating the function of controlling the luminance of lighting in a lighting device using LEDs. However, the technology disclosed in this Korean patent application publication is defective in that errors occur due to interference between signals that are received during visible light communication.
As a result, there is a pressing need for new technology for visible light communication.